1. Field of the Invention
The present invention relates to a demodulator, and particularly to improving equalization capabilities when waveform distortion occurs due to fading in a demodulator in a digital radio-communication system.
2. Description of the Related Art
In the development of digital radio-communication systems in recent years, the number of signal points in QAM have been advanced as a means of raising the utilization of radio frequencies. Because digital radio-communication systems are easily influenced by waveform distortion caused by fading, the advancement of the number of signal points in QAM has been accompanied by the development of adaptive equalizers that equalize waveform distortion. One example of such an adaptive equalizer that has been put to practical use is the adaptive time domain equalizer having transversal filter which adaptively controls the tap coefficient of a transversal filter.
One example of a demodulator that employs an all-digital adaptive equalizer (Japanese Patent Laid-open No. 110930/91: Automatic Equalizer) is shown in FIG. 1.
This prior-art demodulator is made up of a rolloff filter (not shown) for waveform shaping, multiplier 10, compressor 11, analog/digital (A/D) converter 12, transversal filter 13, variable expander 14, and tap coefficient control circuit 15. A controlled input signal 1 of an intermediate frequency band or a radio frequency band is multiplied by a regenerated carrier signal 3 at multiplier 10, and a demodulated baseband signal is outputted such that the output level of transversal filter 13 is a fixed level. The demodulated baseband signal is inputted to compressor 11 and compressed at a prescribed compression ratio, and then inputted to A/D converter 12. A rolloff filter is arranged either before or after the multiplier. The compression-ratio of compressor 11 is set so as not to exceed the range of the permissible input level (dynamic range) of the A/D converter even when waveform distortion occurs due to fading in the transmission line. This compression ratio is here defined as post-compression amplitude/pre-compression amplitude!. If the output of compressor 11 is set to the full dynamic range of A/D converter 12 in a state in which there is no waveform distortion due to fading, the demodulator will be unable to perform correct waveform equalizing because, when fading does occur, a signal exceeding the dynamic range will be inputted to A/D converter 12 and nonlinear distortion will occur in A/D conversion.
The AID converted digital signal is next inputted to all-digital transversal filter 13, automatically equalized by a tap coefficient control signal from tap coefficient control circuit 15, and inputted to expander 14. Expander 14 multiplies the input signal by an expansion ratio equal to the inverse of the compression ratio in order to restore the original signal that was compressed by compressor 11, and outputs the result to terminal 2.
Another demodulator is provided with a compression/expansion ratio control circuit (not shown) that varies the compression ratio of compressor 11 and the expansion ratio of expander 14 based on a tap coefficient control signal, and such a demodulator is devised as a method of reducing quantization error during A/D conversion in a steady state, and preventing saturation of the A/D converter during A/D conversion in a fading state.
As described hereinabove, in an adaptive equalizer of the prior art, the input signal level of multiplier 10 must be set to a low value so as not to exceed the dynamic range when waveform distortion occurs as a result of fading. However, reducing the input signal level introduces the problem that the S/N ratio of the demodulator is also reduced.